Environmental groups on Tuesday sued the city of Chicago's water treatment authority, charging its sewage promotes algae growth that is choking Midwestern rivers and contributes to the Gulf of Mexico's "dead zone."

The federal lawsuit demands Chicago's Metropolitan Water Reclamation District deal with frequent runoff of untreated sewage into local waterways during storms that cause problems downstream.

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The suit also demands the district remove some phosphorous from its treated effluent that is the largest single contributor to the oxygen-poor "dead zone" in the Gulf of Mexico at the mouth of the Mississippi River. The oxygen-poor zone where scant life exists covers thousands of square miles (km).

"Other cities are managing to do this without breaking the bank," said Ann Alexander, an attorney with the Natural Resources Defense Council, part of the environmental coalition bringing the lawsuit.

Removing the phosphorous would cost less than $2 per household per month, Alexander said. Milwaukee, Cleveland and other cities along the Great Lakes and Mississippi River are making greater progress than Chicago, she said.

A spokeswoman for the District said the agency has not seen the lawsuit, and could not comment.

However, District spokeswoman Patricia Young said the agency is in discussions with the U.S. Environmental Protection Agency on a consent decree to address the problems.

Alexander said there have been too many delays already, noting that the city dumped untreated sewage combined with storm water runoff into local waterways 55 times last year, a violation of the Clean Water Act. During the heaviest storms, untreated sewage is released into Lake Michigan, the city's source of drinking water.

Since the mid-1970s, the District has been building "Deep Tunnel," which are huge tunnels to transport and store storm water. But the multibillion-dollar project has been stalled because planned reservoirs have not been dug. The next reservoir will not be ready to accept overflows until 2014 and the project may not be completed 15 years after that.